The present invention relates generally to programmable read only memories (PROM) and more specifically to programmable read only memories which use a vertical fuse technique.
PROM array or matrices generally include a plurality of cells each including a fusible element and an isolating device, for example, a diode, connected in series between a bit line and a word line. Programming of the cell includes destroying the fusible element to store an open logic state therein versus the connected logic state. Initially the fusible element was a thin metal wire, for example, nichrome, which would melt on the passage of large currents therethrough. With the development of integrated circuits, the designers attention was concentrated on providing a totally integrated circuit technique to PROMS. A typical example of totally integrated circuit PROM arrays is shown in U.S. Pat. No. 3,641,516 to Castrucci, et al., wherein two back-to-back diodes connected in series between the word and bit line, are formed laterally in a junction-isolated region. The cell is programmed by applying voltage across one of the diodes using a special third metal contact to produce a fusing current which causes a metal-semiconductor alloy to form at the surface of the substrate and short out the reverse diode.
The equivalency of the back-to-back diode to a bipolar transistor is discussed in U.S. Pat. No. 3,733,690 to Rizzi et al. The word and bit lines are connected to the emitter and collector, respectively. A surface short between the base and the collector or the base and emitter along the surface of the semiconductor material is accomplished by migration of the contact material upon the application of electrical power to the cell. The programming of two serial connected, serial opposed Schottky diodes formed in a junction isolated region using surface migration or shorts is described in U.S. Pat. No. 3,781,825 to Burker et al.
In these systems using surface migration, the bipolar transistor or back-to-back diodes are formed having three laterally spaced surface contacts in a junction isolated region. The use of the junction isolation between cells requires extensive surface area. Similarly, three surface contacts also increases the amount of surface area needed to form a matrix. Thus, although these systems provide reduction in manufacturing expense and materials by using integrated techniques versus metal fuses, the requirement of junction isolation between cells as well as the amount of surface area needed for three surface contacts undesirably increases the size of the matrix. Thus, there exists a need for a PROM cell which minimizes the amount of area required.